1. Field of the Invention
The present invention relates to a photomask and a method for manufacturing the same, and particularly to a half-tone type phase-shifting mask and a method for manufacturing the same.
2. Description of the Related Art
In recent years, problems in the photolithographic step applied in a process for manufacturing semiconductors are becoming increasingly marked. With the progress of fineness in a semiconductor device, demands for the fineness in the photolithographic step are increasing. In this respect, such fineness has already progressed up to halfpitch (hp)=45 nm in the design rule of a leading-edge device. Hence, exposure achieved by combining liquid immersion exposure with polarization illumination has managed to realize the fineness. In these circumstances, light transmittance 6% in a half-tone mask which has been heretofore considered optimum is no longer optimum in a fine pattern of hp=50 nm or less. In this connection, there is described that a lower light transmittance of 2% or less is the most appropriate in a pattern of hp=45 nm in T. Sato, et al., “Impact of polarization for an attenuated phase shift mask with ArF hyper-NA lithography” Proceedings of SPIE, vol. 5754, pp 1063-1069.
This is because intensity balance between zero-order diffracted light and primary diffracted light of the diffracted light passed through the mask is affected by the space structure of the mask to disrupt the balance. It is caused by the fact that the intensity of zero-order diffracted light weakens relatively with decrease in the dimension of the mask. More specifically, a half-tone type phase-shifting mask becomes the most effective at the point at which intensities of the zero-order diffracted light and the primary diffracted light come to be substantially equal to each other. The transmittance at which the intensity ratio of the zero-order diffracted light and the primary diffracted light comes to be optimum becomes lower with reduction in the size in a fine pattern.
However, also in a photomask for manufacturing a semiconductor integrated circuit device having an integrated circuit pattern of hp=45 nm (called “device pattern” in the present specification), there are a pattern region of hp=45 nm and also pattern regions of more than hp=45 nm. This suggests that there arise circumstances in which the optimum light transmittance of a half-tone pattern differs depending on a dimension of the device pattern in a photomask surface.
Incidentally, since the light transmittance of the opening differs depending on the dimension of the opening (phenomenon that the smaller opening brings about usually the smaller light transmittance in a dimension being substantially the same as that of the wavelength is well-known) in a Levenson type phase-shifting mask, a method for lowering the light transmittance in a region in which the opening is large in conformity to a fine pattern where the light transmittance becomes the smallest is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-118614.